Plenum cable is used to carry power through ducts which are used to ventilate, for example, high-rise buildings. While a fire occurring in these ducts can be dangerous in its own right, such a conflagration is especially insidious because the smoke and other gases resulting from the fire are transported through the ducts throughout the building, even to parts quite remote from the blaze. In some cases, colorless and odorless gases can invade sleeping quarters housing unsuspecting people.
The cable used in plenums is generally constructed of a metal conductor insulated with a polymeric material. These elements are generally twisted to form a core and are protected by another polymeric sheath or jacket material. In certain cases, added protection is afforded by inserting a wrap between the core and the sheath.
To mitigate the dangers of a duct fire, it is important that the cable jacketing is not only flame retardant, but, on burning, produces a minimum amount of smoke, emits low levels of corrosive gases and is self-extinguishing. Further, the cable should not raise installation problems caused by materials, which are toxic to the touch.
Ethylene polymers are considered to be deficient insofar as flame retardance or flame resistance is concerned. One method for increasing flame resistance in these polymers is to formulate using metal hydroxides as the flame retardant additive. Both aluminum hydroxide and magnesium hydroxide have been used in this way. Generally, these hydroxides are viewed as equivalents as flame retardants in this context. Aluminum hydroxide is preferred on a cost/performance basis, however. Thus, more than ninety-five percent of all hydrate based commercial formulations use aluminum hydroxide.
In spite of the cost advantage of aluminum hydroxide, there is a strong thrust towards improving the flame resistance of plenum cable as well as its smoke, corrosion, self-extinguishing, and toxic characteristics because of the vulnerability of the occupants of buildings containing plenums through which air is transported for heating and cooling.